1. Field of the Invention
The present invention relates to fabrication of lightweight aircraft transparencies, particularly those that have to be both shaped and laminated. Specifically, the present invention relates to method and apparatus for fabricating lightweight aircraft transparencies that require a minimum of expensive equipment and that consume a minimum of parts during the fabrication of such transparencies.
2. Description of the Technology and Patents of Interest
This invention relates to the fabrication of shaped, laminated lightweight plastic transparencies, particularly those comprising at least one sheet of acrylic plastic resin. The acrylic resin may be laminated to additional sheets of acrylic plastic or polycarbonate plastic to form a laminated transparency especially suitable for aircraft.
The transparencies for aircraft are required to fit within a frame and have to be shaped to fit within the frame. In addition, it is desirable for good optics that any surface of the acrylic resin sheet that forms an exterior or interior surface of an aircraft transparency be as free from surface marks as possible.
Acrylic resin is used in laminated transparencies for aircraft where glass is unsafe, too heavy, or otherwise impractical. Cast polymethyl methacrylate, which is concurrently marketed under such trademarks as PLEXIGLAS by Rohm and Hass and LUCITE by DuPont, is a typical acrylic resin that has several characteristics which make it ideally suited for aircraft transparencies, such as canopies and windows for lightweight aircraft. For example, it is considerably lighter than glass, yet is quite strong at ambient temperatures. Moreover, while acrylic resin is relatively soft compared to glass and therefore scratches more readily than glass, the scratches are easily removed merely by polishing the marred area with common polishing compounds. Furthermore, acrylic resin is fairly stable from a chemical standpoint. It is relatively unaffected by most solvents. Furthermore, acrylic resins accept ultraviolet light stabilizers without significantly impairing their light transmitting characteristics.
For the above reasons, acrylic resin and particularly cast polymethyl methacrylate, has been used as an outer layer of either monolithic or laminated cured aircraft transparencies, particularly for lightweight planes which fly at relatively low altitudes and at relatively low speeds. Under such circumstances, the acrylic windows, windshields or canopies of aircraft do not develop a temperature above the softening point of the acrylic resin, which is in the vicinity of 225.degree. F. (107.degree. C.).
Bent laminated transparencies for aircraft have been produced using relatively expensive equipment such as high pressure autoclaves and/or presses. Furthermore, the relatively rigid components of lightweight laminated transparencies for aircraft, such as sheets of acrylic plastic and/or polycarbonate sheets, have been preformed in a forming operation followed by assembling and laminating the preformed parts to form laminated shaped transparencies. It would be desirable to eliminate such high cost equipment, and it would also be considered an improvement in efficiency of operation if it were possible to simultaneously laminate an assembly while it is being shaped.
In laminating glass plastic assemblies, it has been customary according to one method to enclose the assembly to be laminated so that a peripheral ring formed of a split tube of flexible material clamps against the marginal portion of the top and bottom surfaces of the assembly to form a peripheral evacuating chamber. The evacuating chamber is connected to a source of vacuum so as to remove air and other fluids entrapped at the interfaces between the components of the assembly to be laminated. The assemblies are heated to help bond the components at least preliminarily prior to a final laminating step in an expensive autoclave. While such prepressing is suitable for laminating glass to plastic interlayer materials, when a lower softening point material such as acrylic plastic or polycarbonate plastic is used instead of the glass sheets in the assembly to be laminated, the compressive force of the lips of the evacuating ring causes the relatively rigid outer plies of polycarbonate or plastic to become distorted and develop a frame of optical distortion. Such a frame is undesirable.
Other laminating techniques involve the use of flexible bags within which the elements are assembled and laminated. The flexible laminating bags must be destroyed after one laminating cycle in order to remove the laminated assembly after the assembly has been subjected to elevated pressure and temperature while within the evacuated and sealed laminating bag, so that it is impractical to use the laminating bags more than once. Furthermore, such laminating bags are expensive, and it would be desirable to develop a method that avoids such laminating bags. In addition, it is impossible to evacuate such laminating bags completely so that some residual air remains entrapped therewithin after a laminating operation is ended. Removal of this residual entrapped air remained a problem at the time of this invention.
Various patents disclose the state of the art and will now be described.
U.S. Pat. No. 2,142,445 to Helwig shapes a sheet of thermoplastic material against a concave mold cavity by applying hot liquid under pressure against a surface of a clamped sheet to stretch the sheet against the concave mold cavity and chilling the sheet as it approaches the cavity. The shaping is confined to a monolithic sheet and no laminating is involved.
U.S. Pat. No. 2,948,645 to Keim discloses the use of a peripheral evacuation chamber to remove fluid from between the interfacial surfaces of an assembly containing a pair of glass sheets on opposite sides of a flexible sheet of interlayer material during the preliminary stage of a laminating operation. The lips of the peripheral evacuation chamber are stressed to impart a clamping pressure about the perimeter of the assembly to be laminated. This clamping pressure causes the interlayer to extrude from the margin of the assembly to form a frame of optical distortion that is undesirable in aircraft transparencies. Therefore, the peripheral chamber is removed before the final lamination in an autoclave.
Other patents showing similar peripheral evacuation means during a glass sheet lamination operation include U.S. Pat. No. 2,992,953 to Talburtt, U.S. Pat. No. 3,074,466 to Little, U.S. Pat. No. 4,281,296 to Jameson and U.S. Pat. No. 3,508,996 to Hill. A peripheral evacuation technique using a tight gas seal tightly secured to the edge of the assembly during the laminating of polymethyl methacrylate is shown in U.S. Pat. No. 3,284,263 to Jamet. U.S. Pat. No. 3,912,542 to Hirano et al. uses peripheral evacuation and heat to laminate glass sheets to a synthetic resin in an assembly covered by a flexible diaphragm clamped to a support for said assembly. There is no sheet of interlayer in the assembly to be laminated or shaping of the assembly during its lamination in Hirano et al.
U.S. Pat. No. 3,769,133 to Halberschmidt et al. inserts an assembly to be laminated within a flexible bag, the bag is inserted within a chamber in communication with the chamber, the chamber and bag are evacuated to degas the assembly, the bag is sealed while the chamber is at low pressure and then the sealed bag and its contents are exposed to an elevated pressure and temperature to laminate the assembly within the bag. No shaping takes place during the laminating operation and superatmospheric pressure is used to complete the lamination. This requires expensive equipment for elevated pressure application.
U.S. Pat. No. 3,852,136 to Plumat et al. applies a peripheral evacuation tube around the periphery of a plurality of assemblies that are laminated in unison within a chamber or a series of chambers in which the pressure cycle differs from the evacuation pressure applied to the interfacial surfaces of the assemblies through the peripheries of the latter. No shaping is involved with the laminating technique of this patent and superatmospheric pressure is applied.
U.S. Pat. No. 4,180,426 to Oustin et al. evacuates an assembly to be laminated while the assembly is enclosed in a sack and the sack enclosed within a chamber, where pressure and temperature are subjected to a cycle suitable for laminating. No shaping is involved in the laminating process.
U.S. Pat. No. 4,231,827 to Wilson et al. sag bends individual or fused sheets to be laminated to approximate shape and then assembles the sag bent sheets over a flexible blanket of polyurethane foam or felt resting over a vacuum type mold, and a flexible diaphragm is applied over the sag bent sheet and the vacuum mold to form the roof of an evacuating chamber around the assembly. The polyurethane foam blanket separates the sheets to be shaped from direct contact with any imperfections in the shaping surface of the vacuum mold. After vacuum forming the layers of the assembly to preliminary shape, the shaped acrylic layers are assembled with interlayers between shaped polycarbonate pressing plates to form a laminating assembly, and the latter is taped and inserted in a flexible laminating bag. The latter is evacuated and sealed and the assembly within the bag between the pressing plates is laminated in an autoclave. The shaping and laminating steps are consecutive, not simultaneous, and the process requires an expensive autoclave.
U.S. Pat. No. 4,287,015 to Danner forms a vacuum bag by applying a flexible sheet of stretchable material over and beyond raised ribbing surrounding an assembly to be laminated to encase the assembly in a vacuum bag. Reusable fastening means is provided to attach the flexible sheet to a support base beyond the ribbing. The bag is evacuated and the assembly within the bag is laminated but not changed in shape.
The art of fabricating lightweight aircraft transparencies at the time of the present invention was in need of low cost apparatus and a relatively low cost operation involving the use of reusable materials that would simultaneously laminate and shape the components of an assembly to be fabricated into a lightweight laminated transparency for aircraft.